Post consumer plastics, such as polyester resins, can be recycled by melt fabrication to produce articles which can serve in utilities usually less demanding than the same articles molded from virgin resin. The reason for this less demanding utility may arise from the presence of contaminants accompanying the post consumer plastic. Efforts are made to remove all contaminants, but this is an elusive goal under the current state of recycle technology.
It thus becomes desirable to incorporate resin modifiers in the recycle resin which will upgrade its properties. For recycle polyester resins, a highly desirable group of modifiers are the ethylene random copolymer tougheners such as disclosed in U.S. Pat. No. 4,172,859 (Epstein I). In most cases, these modifiers are incompatible with the polyester matrix resin making it difficult to get the fine dispersion of modifier into the matrix resin that is necessary for the modifier to upgrade the properties of the matrix resin rather than detract therefrom or affect them so modestly that the modification is economically impractical. The same situation exists for virgin resins when the modifier resin is incompatible therewith.
U.S. Pat. Nos. 4,172,859 (Epstein I) and 4,174,358 (Epstein II) disclose the toughening of polyester and polyamide resins, respectively, by the incorporation of relatively low modulus random copolymers in the polyester or polyamide matrix. The methods of incorporation disclosed are (i) melt blending in a twin screw extruder or other conventional plasticating device, such as a Brabender or Banbury mill, (ii) blending by coprecipitation from solution, and (iii) blending or dry mixing of the components, followed by melt fabrication of the dry mixture by extrusion (Epstein I, col. 10, 1. 37-47). In the case of melt blending, further details on the use of the twin screw extruder are disclosed, ending with the extruder producing an extrudate which is cooled in a water bath, cut, dried, and molded into test pieces (Epstein I, col. 10, 1. 48-57). The cutting step produces molding pellets, generally having at least one dimension which is at least 2 mm. The molding step involves the use of an injection molding machine (Epstein I, col. 12, 1. 19-22), i.e., the molding pellets are fed to the injection molding machine for fabrication into the test bars. Epstein II sometimes uses other plasticating apparatus (Brabender, roll mill) in place of the twin screw extruder.
In both Epstein I and Epstein II, the injection molded test bars were prepared in two steps, first, compounding of the random copolymer into the matrix resin to form molding pellets (hereinafter referred to as pre-compounding), followed by injection molding to form articles as the second step. This sequence of steps was selected in the Epstein patents because of the need to achieve a very fine dispersion of the random copolymer within the resin matrix in order to realize optimum toughening in the molded articles, e.g., impact test bars. The patents disclose the fineness of this dispersion desired, e.g., random copolymer particle size of 0.01 to 3 microns, but preferably 0.02 to 1 micron, within the matrix (Epstein I, col. 5, 1. 14-15). The second step, injection molding, takes the molding pellets, melts them and injects the molten resin into the test bar mold. The dispersion is accomplished in the pre-compounding step and the fabrication is accomplished in the injection molding step. Example 168 of Epstein II departs from this combination of operations by extruding a film from a blend of 66 nylon with a fumaric acid-grafted EPDM and subjecting the extruded film to stretching or thermoforming.
This has been the commercial practice for toughening polyester and polyamide resins with other more flexible resins which are relatively incompatible with these matrix resins. This is the present commercial practice which offers itself for use to upgrade post consumer plastics. Unfortunately, this two step preparation process has the disadvantage of raising the cost of the molded articles.
It would be desirable if the pre-compounding step could be eliminated and the matrix resin and toughening resin be brought together for the first time as the feed to the injection molding machine, so that articles could be directly fabricated from the blend components.
Unfortunately, injection molding, such as that used in common single-stage injection molding machines does not lend itself to making a fine dispersion of incompatible resins within a matrix resin, hence the need heretofore for the pre-compounding step. Such injection molding machines use a single screw which both reciprocates and rotates within a barrel in the following sequence of steps which constitute the molding cycle:
(i) screw forward or injection (fill) time PA0 (ii) hold time PA0 (iii) mold open (eject) time. PA0 "A reasonable morphology was achieved only with a combination of Ingen Hausz, Maddock and pineapple sections and a high back pressure. However, the thermal load on the polymer (the resulting melt temperature exceeded the barrel temperature over 80.degree.) caused degradation of the PP, resulting in slightly poorer mechanical properties than those of the commercial blend." PA0 (p. 481).
During the screw forward time, the screw rams towards the injection port (nozzle) of the machine to force molten resin into the mold. Also included in this step is the time the screw is held in the forward position to keep the mold full of molten resin as the molded article starts to solidify.
During the hold time, the screw rotates and retracts under the pressure of the molten resin being forced by the screw into the forward end of the barrel, i.e., adjacent the injection port of the barrel. During this rotation, the resin feed to the injection molding machine becomes melted and transported into this injection position. Normally, when the screw retracts to a certain point, this means the forward end of the barrel is filled with the desired amount of molten resin and the screw stops rotating. Additional hold time is typically taken up with the screw positioned stationary in the retracted position until the molded article has cooled sufficiently.
During the mold opening step of the cycle, the screw remains stationary and retracted while the mold opens and the molded article is removed from the mold.
A typical molding cycle might take 43 seconds, consisting of 20 seconds screw forward time, 20 seconds hold time, and 3 seconds mold open time. Of the 20 seconds hold time, typically only a portion of it is screw rotation time, e.g., 5 seconds whereby it is apparent that the screw rotates for only a small fraction of the time of molding cycle.
Faced with this fact, pre-compounding has served as the standard for resin feed of incompatible resins to injection molding machines.
C.P.J.M.Verbraak and H. E. H. Meijer, "Screw Design in Injection Molding", Polymer Engineering and Science. Vol. 29, No. 7, pp. 479-487 (April, 1989) discloses the insufficient plasticating capacity of injection molding with high capacity (p. 479) and the testing of screw designs in injection molding using screw sections taken from continuous extrusion practice (sentence beginning p. 479-480). The article reports the testing of both distributive mixing and dispersive mixing capability. In distributive mixing, polyethylene is blended with color masterbatch to determine color distribution within the resultant blend obtained with various screw designs. Color masterbatch is normally made from colorant dispersed in polymer which is the same or is at least miscible with the resin being colored, so that uniform coloring of the resin can occur in injection molding. In dispersive mixing, ethylene-propylene-diene (EPDM) elastomer is blended with polypropylene (PP) using various screw designs in an injection molding machine. In dispersive mixing, the elastomer may remain a separate dispersed phase within the polymer matrix or become dissolved in the polymer matrix if sufficient compatibility (miscibility) exists.
Verbraak et al. reports testing eight different screws for dispersive mixing and summarizes the results of this testing as follows:
This "reasonable morphology" is based on examinations at only 40X and 160X magnification. While "globs" of EPDM are visible at these low magnifications, the degree of dispersion necessary to obtain optimum toughening is not visible at these magnifications.
Even with the degree of dispersion or miscibility of the EPDM in the polypropylene obtained, this was at the expense of polymer degradation because of overheating of the polymer within the barrel. This result required greatly increased back pressure on the screw, i.e., pressure applied to retard the retraction of a screw during the hold time portion of the overall injection molding cycle. As indicated in Table 3, for the particular screw in question, 1H6M5PA, the back pressure was increased from 0 bar to 150 bar, which increased plasticating time from 8.3 to 17.7 seconds.
The result of the dispersive mixing reported in terms of IZOD impact strength at -40.degree. C. in kJ/m.sup.2 in Table 3 is rather uninspiring. The maximum impact strength achieved was 8.6 kJ/m.sup.2 for the polypropylene/EPDM blend, which corresponds to 87.4 J/m when converting the test result to the procedure to ASTM D-256. D'Orazio et al., Polymer Engineering and Science, June, 1982 reports the room temperature impact strength of polypropylene/EPDM in an 85/15 wt. ratio to be only 2.6 J/cm.sup.2 which corresponds to 117.4 J/m when converting the test result to the procedure of ASTM D-256. In other words, the blend of polypropylene/EPDM is not very tough to begin with, especially from the point of view of applications demanding room temperature impact toughness of 300 J/m and higher. Verbraak et al. does not disclose his dispersive mixing to achieve this desired toughness result.
In Injection Molding Machines, by A. Whelan, published by Elsevier Applied Science Publishers, Ltd., Essex, England (1984) the use of back pressure in injection molding machines is generally described on pages 398-401. Back pressure is the pressure the screw must overcome in order to retract (p. 398). Back pressure is usually adjusted as low as possible which yields the result of a well-compacted melt which is free from bubbles or voids (p. 399). The use of increased back pressure will result in improved mixing, but accompanied by disadvantages of long screw recovery times, high pressures on the resin melt which may result in nozzle drooling, and increased wear of the injection molding machine (p. 399).
Du Pont Information Bulletin A-88012 (1973) provides information on the use of back pressure in injection molding. Back pressure is disclosed to be helpful for acrylic resins as a way of preventing air pick-up in the screw which cause black streaks in the molded part. For polyamide (Zytel.RTM.) and polyoxymethylene (Delrin.RTM.), back pressure may help produce a more uniform melt temperature and in color mixing, but is not needed for most molding and could cause nozzle leakage.
EPO 0 340 873 Al discloses a mixing device with distributive mixing action for an extruder and injection molding machine, which is useful for mixing viscous materials such as melted plastics and rubber, materials such as soap and clay in addition to foodstuffs such as dough and margarine. Product literature on the mixing device of this publication, entitled "Twente Mixing Ring", published by the University of Twente, and understood to have been printed in November, 1989, discloses various mixing applications and test results including the disclosure of "fewer unmelted particles". The existence of unmelted particles indicates that this device is intended for distributive mixing.
U.S. Pat. No. 4,912,167 discloses an improvement in melt blends of polyester resin with an epoxide copolymer, the improvement involving the incorporation of metal salts of certain acids or certain carboxyl-containing polymers, the metal salts being selected from the group consisting of Al.sup.+++, Cd.sup.++, Co.sup.++, Cu.sup.++, Fe.sup.++, In.sup.+++, Mn.sup.++, Nd.sup.+++, Sb.sup.+++, Sn.sup.++, and Zn.sup.++. The nature of this improvement is disclosed to be increased melt strength and increased melt viscosity for the blends, enhancing the blow moldability of the blends. The blow molding fabrication is disclosed to be carried out in two steps, first the ingredients are melt blended and pelletized and these pellets are then fed to an extrusion blow molding machine (Col. 4, 1. 36-45).
In summary, injection molding has been used to obtain distributive mixing of colorant within polymer, while preparing the polymer for injection molding. Verbraak et al. discloses an unsuccessful attempt to perform dispersive mixing in an injection molding machine. The polymers used in Verbraak et al., polypropylene and ethylene-propylene-diene elastomer, are fairly compatible as indicated by the similarity of their solubility parameters of 16.0 (J/cm.sup.3).sup.1/2 and 16.5 (J/cm.sup.3).sup.1/2, respectively.
There still exists the need to be able to more economically injection mold a blend of incompatible thermoplastic resins, which would be especially useful for upgrading post consumer plastics, by eliminating the need for pre-compounding of the resins.